The invention is directed to a magnetic lens for focusing a charged particle beam generated by an instrument to a very small spot for deriving characteristics of a sample and, in particular, to create a magnetic flux pattern which provides improved high resolution.
Various instruments are known which rely on interaction of charged particles from a sample to derive characteristics of the sample. Examples of such instruments are an electron microscope and a focused ion beam microscope. A focused beam of charged particles is also used in a machine for conducting electron beam lithography.
For facilitating the description of the present invention, it will be explained in connection with a scanning electron microscope (xe2x80x9cSEMxe2x80x9d). However, it should be understood that the invention is not limited to an SEM and can be applied by one with ordinary skill in the art to instruments and machines such as those mentioned above which require a focused beam of charged particles.
An SEM operates by generating a primary scanning electron beam that impacts a sample, a surface of which is being imaged. As a result, backscattered and secondary electrons are emitted from the sample surface and collected by a detector which is arranged near the surface of the sample. The detector generates a signal from the electron emission collected from the sample surface as it is exposed to the electron beam. The signal from the detector is used to display an image of the surface on a video screen.
A typical arrangement of the main components of an SEM is schematically shown in FIG. 1. Electron source 2 generates an electron beam 3 which is directed through aligned openings at opposite ends of tube 4 toward sample 5. Detector 6 collects electrons emitted from sample 5. Beam 3 passes through opening 8 in detector 6. Beam 3 is controlled by stigmation coils 7, alignment coils 9, scan coils 11a and 11b, and lens 13. The function of these components is well known. Briefly, stigmation coils 7 are used to correct the shape of the beam. Alignment coils 9 are used to align the beam through the tube 4. Scan coils 11a and 11b deflect electron beam 3 in two directions, respectively, such as along an x-direction and a y-direction in a plane perpendicular to the beam direction. SEM""s can contain more than one of any of these components.
Electromagnetic lens 13 is provided for focusing of the beam 3 to a very small spot to enable high resolution imaging. One type of lens 13 is an immersion lens. U.S. Pat. No. 5,493,116 discloses an immersion lens, and that lens is shown schematically in FIGS. 1 and 2 hereof. It includes a toroidal, channel-shaped magnetic polepiece 14 with a lens inner pole 15 and a lens outer pole 17, and a winding 19 inside the channel.
One characteristic of an SEM lens is its electron-optical working distance (xe2x80x9cE.O.xe2x80x9d). The E.O. refers to the distance between the surface plane of sample 5 and a plane corresponding to a region of maximum flux density of the lens. The region of maximum flux density for lens 13 is located at plane 22. The E.O. is described as being slightly negative by approximately xe2x88x921 mm, so that the plane of sample 5 is above the plane 22. This configuration is alleged to have the beneficial result of considerably increasing the collection efficiency of low-yield backscattered electrons because electrons are swept by this slightly negative E.O. onto the detector (or detectors), such as the electron shown as having an initial trajectory along path 20, which is at a significant angle from normal, but is deflected and reaches the detector via deflected trajectory 21 (see FIG. 2).
A shortcoming of this prior art approach, however, is that the magnetic field, as shown in FIG. 2, interacts with the sample and anything below the sample in the SEM if they have magnetic properties, such as the x-y stage (not shown) which is used to move the sample to its desired scanning position relative to the electron beam. Such interaction causes the field to become distorted. In fact, it is not as shown in FIG. 2, and this deteriorates the resolution achievable with the instrument. In addition, the flux below the sample serves no useful purpose, but power is consumed to generate it. Power used to create this flux generates heat which then has to be conducted away from the coil winding 19. Furthermore, aberrations in generating the small spot can be minimized by creating a magnetic flux pattern which has a concentration of magnetic field near the sample. Since this prior art approach does not produce such a field, higher aberration coefficients can be expected.
A pinhole lens is another type of magnetic lens known in the prior art for focusing a charged particle beam. In contrast to the immersion lens, the bulk of the magnetic field generated by a pinhole lens is above the sample (i.e. it has a positive E.O.). A shortcoming of this lens is that it has a high focal length which interferes with attaining a high resolution. Also, on-axis and near on-axis electrons cannot pass through this field and, therefore, the detector must be positioned below the lens. This further increases the focal length and exacerbates the difficulty in attaining high resolution. Moreover, a detector located in that position can collect only electrons which are substantially off-axis, thereby losing the other electrons.
One object of the invention is to provide a magnetic lens which produces improved focusing of a beam of charged particles.
Another object of the invention is to provide a magnetic lens which produces improved high resolution imaging.
A further object of the invention is to provide a charged particle lens for imaging which exhibits reduced aberration coefficients.
Yet another object of the invention is to provide a magnetic lens which does not waste power.
Still another object of the present invention is to provide a magnetic lens having a magnetic field which does not interact strongly with the sample and things below the sample.
One other object of the invention is to create desirable lens properties for a magnetic lens while allowing emitted electrons to efficiently reach the detector.
These and other objects are attained in accordance with one aspect of the present invention directed to a magnetic lens for an instrument which directs a charged particle beam toward a sample. A polepiece includes an inner yoke, an outer yoke and a winding. A lens outer pole is secured to the outer yoke and includes a first surface having a first opening defined therein positioned such that the beam passes therethrough. A lens inner pole is secured to the inner yoke and includes a second surface having a second opening defined therein aligned with the first opening, but with a smaller inner diameter.